Method of forming subdensity metal bodies



METHOD OF FORMING SUBDENSITY METAL BODIES Filed Feb. '24. 1954 3 Sheets-Sheet 1 CRUSHER SGREENS OR ox/os A010 0R, s/znvc GRINDER. DEV/CE.

ADMIXTURE cpucmrmron GANGUE NON- OX/DIZINC ATMOSPHERE H 1 HOT HOT SIM PING SUPPORT WORK/NC -WORK'D nznucmc AND s/A/mmvc PRODUCT June 17, 1 958 I QAVANAGH 2,839,397

METHOD OF FORMING SUBDENSITY METAL BODIES Filed Feb. 24, 1954 I S-Shets-Sheet 2 D/FFERFN TIAL oe-mRMAr/ou OEFORMA r/o/v 2 747/ v 111/ Ill/g MM PAT Ic El-CAVANA H.

June 17, 1958 P. E. CAVANAGH 2,839,397

METHOD OF FORMING SUBDENSITY METAL BODIES Filed Feb. 24. 1954 Y a Sheets-Sheet 3 PATRICK E. CAVANAGH' 2,839,397 Patented June 17, 1958 ice METHOD OF FORMING SUBDENSITY METAL BODIES Patrick Edgar Cavanagh, Oakville, Ontario, Canada, assignor to Ontario Research Foundation, Toronto, Ontario, Canada, a corporation of Ontario Original application December 23, 1952, Serial No. 327,575. Divided and this application February 24, 1954, Serial No. 412,307

4 Claims. (Cl. 75222) This invention relates to the manufacture of metal products and particularly to methods adaptable to large scale operations for forming wrought ferrous metal products; for example, sheets, plates, strips, bars, rods and the like, directly from compositions, that comprise previously unreduced oxygen bearing metal compounds or from bodies below theoretical full density of the metal thereof, hereinafter defined as subdensity metal bodies.

This application is a continuation-impart of my applications Serial Number 231,074, filed June 12, 1951 (now abandoned) and of application serial number 232,921, filed June 22, 1951 (now abandoned) and a divisional application of application Serial Number 327,575, filed December 23, 1952 (now United States Patent No. 2,686,118).

By a subdensity metal body, I mean a metal body having a density which is substantially less than the density of the solid material thereof, by reason of voids in the body. It will be understood that such metal body,

may be formed of a single metallic element or of an alloy or mixture of a number of metals with or without other materials. In an important specific sense, the present invention is concerned with ferrous metal bodies, wherein the predominant component is iron, examples being compositions corresponding to various steels, other ferrous alloys and the like.

While the invention relates to the formingof wrought ferrous metal products from subdensity bodies, it is preferred to practice the invention upon that class ofsubdensity body which is formed from controlledidensity metals including techniques associated with manufacture of the latter as set forth in the said co-pending applications. A controlled density metal body may be described as a body produced by complete or partial reduction of oxygen bearing material at temperatures below the melting point of the body, in suitable apparatus'whereby the body is produced and proportioned to a predetermined shape of predetermined dimensions during the reducing process; said oxygen bearing material comprising a compound or compounds of one or more metals which, before reduction, may or may not have been mixed with other metallic and/or non-metallic materials; said body having predetermined surface conditions and a predetermined density between 5 percent and 100 percent of the theoretical full density of the solid materials of said body.

Prior proposals for forming ferrous products directly from oxides or from subdensity iron particles have involved a complex series of operations and have often been directed to the formation, at the outset, of a large ingot or billet of high density. The ingot or billet is sometimes required to be so compacted in formation as to reach the equivalent of full, solid metal density, or is produced in some other special form of lamination or the like, as heretofore believed necessary for passage of the billet through the rolls of a conventional rolling mill.

In contrast to prior methods, the present invention differs largely in concept in having for a main object, the provision of a direct method adaptable to large scale man- 'ufacture for forming hot worked metal products of maximum density, e. g., substantially solid metal density, while eliminating the necessity of forming an ingot of high density or of performing other costly intermediate steps, and wherein the product to be produced may correspond, for example, to the product ordinarily obtained from the final rolls of a rolling mill adapted to work upon an ingot.

Another object of the invention is to provide a method of making sheet steel and similar products of a chosen predetermined thickness directly from a subdensity metal shape proportioned to be reduced to said predetermined thickness when worked by passage through rollsto the point of maximum density. By the term maximum density I mean the maximum density attainable in practice by the body.

The first problem encountered is that-of providing a subdensity metal shape of sufiicient structural unity to endure passage through rolls or other hot working apparatus without disintegration. Three general methods of attempting to provide such a shape are known.

The first class of known method makes use of a pressure technique and a binder for shaping the ore before reduction, the aim being to form a high density sintered product. Three serious disadvantages arise from this approach, namely: it is difiicult for reducing gases to permeate through a highly compacted mass and accordingly reduction times tend to be very long; veryfew iron ores are capable of being reduced after compaction without failure of the compacted shape due to swelling of the ore or undue shrinkage thereof during reduction; the size and proportions of any shape which can be formed by any technique dependent upon relatively large pressures for compaction of the oxide mass are severely limited by both physical and practical conditions as is well known. 7 It, is another object of the invention to provide a nonfriable-subdensity metal shape of structural unity adapted for hot working operations by aiming preferably at the formation of such a shape at low density for the purpose of achieving rapid reduction. In a second class of method for forming a shape of structural unity, loose ore is reduced as in a common sponge iron class of process to provide a mass of metal particles defining no definite shape and of the usual relatively unbonded friable nature and which is highly compacted while hot to form a shape of high density which is subsequently fed to a set of rolls. a

A third class of method for forming a shape of structural unity is exemplified by any of the well known powdered metallurgy techniques wherein powdered metal is compacted while hot or before heating and then sintered to obtain a high density spongy metal article or solid metal article.

7 Attempts to form large ingots by powder metallurgy techniques have not been seriously contemplated for the reason that shape or size factors interfere, limiting practice to the formation of small articles even whenusing very large compacting pressures.

*It is another object of the invention to provide a shaped subdensity metal article'for'med by reducing ore; in a reducing zone without melting while supported to the shape desired and cohering the reduced ore particles to form. a unitary mass of subdensity metal solely by heating the reduced ore .particles to the cohering point in a non-oxidizing atmosphere.

I have discovered that a subdensity mass of metal may be successfully reduced to maximum or full density of the metal itself while substantially unconfined by an enclosing die, mold, or the like, by following techniques in accordance with the invention. In the hot working of subdensity ferrous metals, one criterion according to the invention requires that the hot working belimited to that suflicient to develop less than a permissible amount of differential deformation. By the phrase differential deformation, I mean the difference in elongation in any direction of the core portion of a subdensity metal body compared to the elongation of the surface portions thereof during working. A second criterion requires that if a product is to be wrought so as to have a substantially uniform density throughout, the working, e. g., hot working, should be accomplished by carrying the deformation in one operation (by the application of a continuous force, as in a single rolling pass) from a point where the body has zero difierential deformation or less than the differential deformation permissible under the first criterion above, to a point where the density of the body has sufficiently approached maximum density as again to reach a condition of less than a permissible differential deformation? A third criterion requires that the subdensity metal article he proportioned having regard to the nature of the product to be made.

A still further object of the invention is to provide a process for forming hot worked ferrous products wherein the grain size of the resulting ferrous product is substantially determined by the grain size of the particles-of iron oxide from which the ferrous product is formed, whereby properties normally attributed only to extensively worked ferrous products can be attained with a small amount of working.

A still further object of the invention is to provide a direct method for obtaining a sheet metal product or the like from ores or oxides comprising a process of interrelated steps in each of which the allowable limits of variations are fixed by the desired characteristics of the product. For example, the desired thickness of the end product will determine the roll setting and the dimensions of the controlled density steel slab for a given density of the slab. The desired grain size of the finished sheet will determine the average particle size of, the ore. V

A still further object of the vinvention is to form novel metal products, for example: composite density sandwich metal sheets, strips and/or bars; low density core rods or bars; and metal bodies of manifestly different nature and character than heretofore known.

Other objects of the invention will be appreciated by a study of the following specification, taken in conjunction with the accompanying drawings:

'In the drawings:

Figure 1 is a flow sheet layout of a preferred process according to the invention;

Figures 2A to 2D illustrate the manner of hot working a subdensity ferrous article according to the invention;

Figures 3A to 3D illustrate the manner of hot working a modified form of slab according to the invention to obtain a clean edge condition on a strip, bar or sheet product;

Figures 4A to 4B illustrate methods of hot working a subdensity ferrous article according to the invention;

Figure 5 is a sectional view of apparatus for forming a shaped cohered subdensity articlegand Figure 6 is a diagrammatic perspective of the formation of a subdensity metal body and a manner of working same.

l Referring to the drawings, the concept of the general process will be appreciated from an examination of Figure 1, wherein it will be apparent that an iron oxide is crushed and/ or ground, as may be required, and sized by suitable screens or other sizing device to obtain a preferred particle size distribution and bulk density in the oxide to be used in the present process inaccordance with the requirements of shrinking and swelling characteristics of the oxide during reduction, grain size in the final product, rate of reduction or permeability, of the oxide mass to the passage of reducing gases, and the desired density and strength of the subdensity metal shaped article produced for the hot working techniques; disclosed herein. Silica and other gangue constituents may be removed from the chosen form of'iron oxide material preferably by dry concentration, for example, by a suitable concentrator such as a magnetic concentrator of the class disclosed in Swedish Patent No. 120,710 granted November 27, 1947 to E; H. HaEketorp, et a1.

Following concentration, the sized and concentrated oxide iriay have admixed therewith, finely divided alloying constituents or foreign bodies in accordance with the requirements of the product to be produced.

' The sized and concentrated oxide, with or without admixture, is loaded into a shaping support in the direct forming apparatus, which support is of a nature defining the shape of the subdensity article produced therein during the reduction process. The present process does not rely at this stage upon the application of positive pressure techniques such as extrusion or compaction of the ore by large pressures. The oxide may be freely loaded into the shaping support, this particular technique permitting the present process to be operative with an extremely wide range of ferrous oxide materials which otherwise would react unfavourably if unduly compacted, due to shrinkage or swelling characteristics.

While in the shaping support, the oxide is' heated and subjected to a reducing gas in the reducing chamber as indicated and the reduced oxide particles are heated to a sufiicient temperature to form a cohered subdensity article of sufficient structural unity permitting removal from the shaping support and of sufficient strength to be handled as a non-friable cohered body of predetermined dimensional characteristics adapted to be hot worked in the hot working apparatus. It is at present preferred that the strong coherence of the shaped body be achieved solely by heating it to unite its metallic composition, e. g., at least to a temperature for sin-tering its particles or portions strongly together. It will-be understood that in practice, the reducing and cohering operations may be at least in part simultaneously effected; for example, the last stages of a desired extent of reduction may be performed While heating is being continued at a temperature for effectuating or completing the described coherence.

The hot working techniques of the invention may be discussed with reference to Figures 2 to 4. The subdensity ferrous metal shape 10 at Figure 2A may be hot worked to compact the surfaces thereof to maximum density as indicated by the maximum density outer layer 11 at B surrounding the lower density core 12. I have found that it is permissible to so compact the outer regions of such body Within the limits of an elongation giving rise to stresses between the outer dense skin and the inner low density core of a limited value. If the structure shownat B in Figure 2 is reduced in diameter by further hot working to a point where the skin 13 is of substantial thickness as compared with the diameter of the core 14 as at C, then the elongation of the skin 13, if in excess of about 10 percent, may give rise to undue stresses, causing failure imperfections 15 in the skin. It is for this reason that the ordinary swaging machine or rolling mill practice is not operative to form a maximum density rod from a low density subdensity structure unless methods are modified to follow the methods of the present invention.

I have found that a section of entire maximum density as at 16 in Figure 2D having substantially no voids, as would be the case of an ordinary steel rod or the like, may be formed by working the subdensity article shown at 2A to maximum density under the action of a continuously applied force, that is, in one operation through aswaging device. i

In forming strip or sheet of maximum density, the subdensity metal slab must be of predetermined shape and dimensions to attain the desired results. For example, a subdensity ferrous article 17. shown in Figure 3A may be E ard. n one h pas h u h the {QHQIOLfQ mlh 17 is of insufiic ent aspect ratio, i. e., the ratio of width to thickness is too small, the product 18 may have edge faults 19.- In many cases, small edge faults mayl be eliminated-by edge trimming of the resulting sheet. "-In' general, however, edge faults arising from 'thepropOrtion ing of the subdensity slab may be avoided as indicated; in Figure 3G by sizing the slahsuch as slab 20 to. a width greater than ten times the thickness. As indicated in Figure 31), a maximum density product 21 of more reguular edge form will result. A subdensity metal. slab aspect rat-iojof greater than fifteen is preferred-as giving satisfactory results in most instancesy regardless. of the particular. density of the slab which is to. be worked to maximum density. I V 7,

Edge fault problems derived from slab proportions will not ordinarily arise in the practice of the invention to form a wide sheet of desired thickness at the point of maximum density of the slab. Accordingly, the proportions of the slab will primarily be determined by other considerations. I have ascertained that slab proportions for purposes of the invention are derived from "a consider ation of relative volumes according to the relation maximum density product 18 f Figure 33. If the article Vs Drnaxv Vminf Ds where Vs is the volume of the subdensity slab;

Ds is the density of the subdensity slab; Vmin is the volume of the maximum density product, as it comes from the rolls;

Dmax is the density of the maximum density product.

Dmax.Vmin

- D8 =20 cubic units The volume of the slab sheet. For example, in Figures 4A to 4D," the longitudi nal section of ferrous 'metal bodies is illustrated. In Figure 4A, 'a subdensity ferrous metal'article 22 is shown of a specific length y. 'As in the technique discussed with referencefto Figure 2; the subdensity article 22 may be rolled or otherwisecompacted to form skins of maximum density 23 and 24 on either side of a low density v core25-such as by a single pass through'the rolls while the article'is hot, wherein a differential deformation in the case of ferrous products will be of the order of about 5 percent. As indicated, the skin portions may undergo an elongation of a boutlO percent'or less during a rolling operation; however, as shown in Figure 4C,'if the structure of 4B is subjected to further passes through the rolls, undue differential deformation may occur and the skin portions may elongate'toan extent setting up stresses during rolling in excess'of the strength of the skins. "Thus one. may obtain a core 26 of low density having thereover the skins 27 and 28 having faults 29 and 30. After one pass through the rolls t form a product such as in Figure 4B, one risks the condition of Figure 4C density. l h

It will thus be evident that elongations between about 10 percent and about 25 percent represent a substantially difficult range to be avoided in the working of subdensity ferrous. materials in particular.

upon attempting to roll further to for. many ferrous metal products, this will not be found inconsistent by skilled persons employing these general limits in conjunction with factors of safety assuring de- Assurne a subdensity slab sirable results in any circumstance. The degree of perfection required will be dictated by specifications for'the product.

' If' a product of maximum density throughout is re- The elongation in the direction of rolling will be my 30 percent. The elongation in the lateral direction will be say 10 percent.

The length of the slab may then be Ls =7.7 unlts The width of the slab may be units The thickness of the slab may be Slab dimensions 7.7 4.54 0.57.

Although the aspect ratio of such a slab is less than the preferred minimum, there is a possibility of obtaining satisfactory edge conditionsassuming regular slab edges, good rolling practice and the usual amount of edge finishing or trimming in the final product. 7

While proportioning techniques set forth herein apply largely to methods of forming a product of maximum density material in its entirety, the same techniques generally apply to portions of a slab which maybe reduced.

article 22 of Figure 4A, to continuously applied force such-as by a single pass through the rolls, while hot, to reduce it to maximum density, at which an elongation of.

about 30% or more will be experienced, as indicated inv the case of the product 31 of Figure 4D. As before mentioned, in a more general sense an important criterion of working requires that if a product is to be wrought so as to have a substantially uniform density throughout, the working, e. g., hot Working, should be accomplishedby carrying the. deformation in one operation '(by thejap plication of a continuous fo'rce,'as"in a 'single'rolling pass) from a point where the body has zero differential deformation or less than the differential deformation permissible. under the first criterion above, to a point where the density of the body has sulficiently approached maximum density as again to reach a condition of less than a permissible differential:deformation." It'will therefore be understood that thesingle application of continuous force is exerted to carry the body to a relatively.

high preferably uniform density and to not more than a predetermined differential deformation. In the formation of a sheet, strip or bar, this technique must be taken into acount, along with the technique disclosed in the discussion of Figure 3. V

I have successfully formed products according to the invention by employing a reheating technique whereinthe subdensity ferrous article, is allowed to cooland is then.

While I believe that differential deformations should not exceed about 5 percent applied to three different p 7 TABLE I TABLE III- Mill Scale s Iron Total Fe -73.6%. p Concentrate Sil 0.48%. I l Total Fe 71.4%.

Bulk density 200lhs./ft. or 5 Sil 0.44%.

i 3.2 gms./cc. Bulk density 194 lbs/ft. or Sieve analysis: Sieve analysis: gins/cc o 3313- +60 0.0 gms 8 1 6()' +80 4 gms am 10 3 +100 9,0 gms E +150 235 150 +200; 215 gms .20 +270 20 5 gms em 27 +325 7 0 e 98.5 gms. 995 gms. Reduction temperature 1980 F. Reduction temperature 1980 F. Holding time (reducing and cohering Holding time (reducing and cohering time in hours) 20. 20 time in hours) 10. Sla'b density 3.0 gms./ec. Slab density 2.1 gins/cc. Slab thickness 0.515 inch. Slab thickness 0.410 inch. Single hot pass reduction 75.7 percent. Single hot pass reduction 80.7%. Sheet thickness 0.125 inch Sheet thickness 0.079 inch. Sheet density 7.5 .gms/cc. Sheet density 7.7 -gms./cc.

' computed) (computed). Cold rolling reduction 70, percent. Cold rolling reduction 46%. Cold rolled sheet thickness 10.038 inch. Cold rolling sheet thickness 0.040 inch. Annealing temperature 1400 F. Annealing temperature 1150 F. Annealing time min. 30 Annealing time 30 min.

TABLE IV Properties of rolled sheet (test data) M111 Scale Old Bed Mag Iron Hot Rolled:

RockwellB Hardness--- 6 67. Bend Tests 210 on Ult.Iens1le 69,000 p.s.i. Elongation (tested) 19? 6%. Grain Size. 7-8. Cold Rolled: r I

RockwellBHarrlness 103 9s 99. Bend Tests. 4 4 50. Ult. Tensile 102,000 p. s.i 96,2001) 5 1 99,000 p. s. l. Elongation (tested)- 1 0 1%. Annealed:

Rockwollll Hardness 56. Bend Tests bent on sell. surf. bent on self. tears bent on self. tears openings. through. through. Ult. Tensile 49,000-. 46,300 7,000. ErioksenDuctilityTest; 4.8

TABLE 11 In the foregoing examples, it was proposed toform 21 Old Bed 0 3 inch wide strip of hot rolled steel about 12 inches Concentrate long and less than inch thick. Accordingly, a sub- Total Fe 70.2%. 5 density metal slab was made from each'of the three iron Bulk density 3.5 gms./cc. oxide materials noted, 3 inches wide and 12 inches long Sieve analysis: and of a low density of about 2 to 3 gms./cc. since such 1.0%. a low density would offer a low resistance to the passage 60 22.7%. of reducing gases. Assuming a slab density of 3 and -100 20.3%. having regard to volume-density relations as set forth -150 +200 15.6%. herein, the slab thickness required to produce a 4; inch 200 +325 12.7%. sheet was about /2 inch, giving a very low aspect ratio 325 27.7%. 60 for the slab 0f6to 1. 1

0% The Mill Scale selected was obtained from a continuous hot strip mill and accordingly was relatively low in Reductiontemperature 1980 F. residual alloys and carbon content. Upon analysis, the Holding time (reducing'and cohering Mill Scale showed 73.6% iron and 0.48% silica.

time in hours) 12%. b0 Since it was desiredin all of the above examples to Sla'b density 2.1 gms./-cc. predetermine the grain size of the resulting rolled product Slab th n 0.410 inch. to a conventional magnitude, a substantially correspond- Single hot pass reduction 79%. ing sizing of the Mill Scale was carried out by grinding Sheet thickness 0.085 inch. 7 in a ball mill to minus 60 mesh screen size. The ground Sheet density 7.7 gms./cc. Mill Scale was screened to provide the particle size dis- 1 (computed). tribution shown in Table I of relatively low bulk density Cold rolling reduction 40%. consistent with the delivery of a relatively low density Cold rolled sheet thickness; 0.046 inch. metal slab and accordinglyconducive to rapid reduc- Anneal ng temperature 1150 F. F tion. The hulk-density of the ground and' sized Mill Annealing time 30min. Scale was determined by pouring it into agraduated flask vibrated for thirty seconds to constant volume after which the volume and weight of the contents were measured and the density calculat :d.

Permeable molds were then made by mixing 3% of a thermo-setting plastic binder with foundry sand as in well-known shell molding techniques. A shell mold was formed with this mixture over a metal pattern heated to 500 F. The finished mold was removed from the pattern and after cooling, was loaded with the prepared Mill Scale. The loosely filled mold was then vibrated to settle the loose oxide as much as possible and a shell mold material cover was placed over the upper end of the loaded mold.

A convenient way of performing the formation of the subdensity article within this mold is illustrated in Figure 5. A plurality of loaded permeable molds 32, having covers 32a were placed, one a cylindrical container 33. A metal sagger of inches inside diameter and 18 inches in height, was used. The saggers were fabricat d from sheet alloy steel (15 percent chrome, percent nickel). The bottoms 34 of the saggers were formed of cast alloy, each of the saggers being coated with a suitable high temperature enamel 35 to reduce oxidative tendencies. in loading the molds into the saggers, a reducing mixture 36 was first placed on the bottom of the saggers and then the molds 32 were placed thereabove spaced each from the next by a layer 37 of the reducing mixture with a final layer 38 of the reducing mixture to a level within 2 inches of the top of the sagger. A one inch layer 39 of loose iron ore of finely divided form was placed over the topmost layer of reduciru mixture to form a seal over the open end 40 of the saggers and to prevent reoxidation of any iron produced in the sagger.

The reducing mixture 36 was thus placed entirely outside the material loaded into the molds and comprised a mixture of coke breeze and limestone wherein the coke breeze was of at least 80% carbon, completely dry and sized to minus 8 mesh screen size; 15% by weight of minus Mt inch limestone was added to the coke breeze to act as a desulphurizer to take up the sulphur from the coke and prevent it from going into the iron particles as they were formed from the oxides being reduced in the sagger. The ratio of coke to oxide in the saggers was of the order of 10 of coke to 1 of oxide by Weight.

The loaded saggers were heated to a reducing temperature by passing them through a tunnel kiln at sufficient speed so that the time at a temperature of 1980 F. was 20 hours. This period of time enabled the reduced oxide particles to cohere together to form a nonfriable cohered mass which was allowed to cool over a period of twenty-eight hours to 200 F. The time at temperature during the reducing and cohering periodwas of a time period which, in conjunction with the average size of the oxide, the particle size distribution thereof, the temperature and the amount of carbon available from the reducing gas for solution in the iron particles produced, delivered a carbon content of 0.12% and a density of 3.0 gms./cc. All of these factors were adjusted to produce a controlled density steel slab.

The reduction of the oxide within the molds and the saggers could have been carried out in any batch type furnace such as a heat treating furnace. Any furnace which will sufiiciently heat a number of such saggers to a temperature between 1900 F. and 2200" F. at the rate of about 100 F. per hour and which will hold the saggers at this temperature for a considerable period of time, would be suitable for processing of the type indicated.

In accordance with the invention and as indicated in Figure 6, a subdensity slab 4-1 of controlled density, predetermined shape and dimensions, is removed from a mold 32 after formation therein by the reducing and cohering process and while heated to a working temperabove the other, in-

ature, may be wrought as at 42 to a uniform density of substantially a maximum value byap'plying a continuous force such as by the rolls 43 set substantially .to

the thickness of the product desired.

In the present example, subdensity metal slabs conforming to the shape of the molds were removed from the saggers after cooling and were reheated in an atmosphere of nitrogen containing about 30% hydrogen in an electric furnace to a temperature of 2100 F. and held for ten minutes.

The rolls of a small mill were set to produce a sheet in one pass of the slab to a thickness of 0.125 inch. After passing the heating slabs through the rolls so adjusted, a reduction of 75.7% was efiiected in one pass delivering a density in the final product of 7.5% being substantially the point of maximum density" of the material being worked upon. The value for density was calculated from the dimensions of the product.

In forming slabs from magnetite concentrates, two representative ores were selected. The concentrate designated Old Bed in the Tables H and IV is a New York State magnetite. The concentrate designated Mag Iron in Tables Til EV is Canadian magnetite, these concentrates being well known to persons familiar with the production of iron and steel.

Procedure followed with Mag Iron and Old Bed ores was the same as with Mill Scale. The screen size distribution and bulk density and analysis of the ores are shown in the tables. In both cases, the ore was magnetically con- 'centrated after grinding to remove most of the gangue.

The slab sizes, were the same and the coke to ore ratio the same as for Mill Scale. The holding time at temperature was 10 hours for Mag Iron and 12 /2 hours 'for the Old Bed; As a result, the density of the slabs produced was 2.1 in both cases and the carbon content was 0.27% for the Mag Iron slab and 0.08% for the Old Bed slab.

The slab thicknesses were 0.410 inch and after rolling reduction of about in a single pass after reheating to 2100 F. in hydrogen plus nitrogen atmospherefthe resulting slabs had a computed density of 7.7. Primarily, due to the low aspect ratio of the slabs before rolling,

edge faulting in the final products occurred.

As indicated in the foregoing tables, all of the sheets .obtained by hot rolling in one pass to maximum density, were then subjected to a cold rolling reduction and annealed. Mechanical tests were carried out for each of the hot rolled, cold rolled and annealed products as outlined in Table IV. Observe that the grain size of the hot rolled products Was between 7 and 8 after one hot pass. Average numbers for grainsize for most hot rolled steels range between about 4 and 8, after a large number of hot passes. These numbers for grain size are-in accordance with the classification of the American Society for Testing Materials, denoting the number of grains per square inch at magnification. It is noteworthy that a comparable grain size to that accomplished in ordinary steels, was obtained by the present process by a control exercised on the sizing of the ore. The structural properties listed in Table IV illustrate the accomplishment ofdesirable structural characteristics by one hot pass of the subdensity metal structure through the rolls to maximum density and also show satisfactory properties after cold rolling and annealing.

' The subdensity articles formed for the purposes of forming the maximum density sections, need notbe of a high density themselves. 7 :contemplates the advantage to be gained by making these subdensity articles of a low density (i. e., less than half the value of maximum density, in some cases as low as one fifth; but sufficient to accomplish a cohered body) so that'the oxide mass may offer relatively low resistance to the passage of reducing gases therethrough, thus permitting 'very short reduction times. Space is provided in the voids of the mass to accept swelling and shrinking characteristics of the oxide particles during reduction.

This invention specifically '11 Such characteristics are discussed in some detail in a paper by the present inventor entitled Pelletizing of iron bearing fines by extrusion and reported in 1950 proceedings, volume 9, page 54 of The American Institute of Mining and Metallurgical Engineers.

While the space provided by the voids generally assists in controlling shrinking and swelling characteristics, it is particularly contemplated according to the present invention, to mix shrinking and swelling ores to cancel out such characteristics to a degree permitting a closer dimensional control than has heretofore been possible with most iron ores. The problems are not critical in the preferred practice of my invention wherein the shape of the subdensity article is determined during the reducing process. However, where subdensity metal shapes are formed directly from the oxide by prior methods, the proportioning of the shapes taught herein for working purposes may not be realized or such prior methods may not be operative with many ores unless a blend of a plurality of ores of different dimensional change during reduction is used in accordance with the present invention.

While certain limits of differential deformation have been discussed herein, it must be appreciated that the permissible differential of deformation between the core and skin portion depends upon the material of the subdensity article and the hot working temperature, as well as the nature of the hot working apparatus. It is conceivable that a permissible differential deformation of percent or more may be achieved under special conditions.

On the other hand, it will be apparent that if the subdensity product is worked to maximum density while confined to limit the elongation and hence the differential deformation of the material upon being compressed to maximum density, then the many faults described herein a may not arise.

By way of recapitulation with respect to rolling and like operations, it has been indicated above to be preferable that in rolling a subdensity slab to form a product having solid metal density the slab be hot rolled with sufficient reduction and elongation in a single pass to carry the body directly from its unworked subdensity state to substantially maximum density. It will be understood, however, that a rolling pass which in the above general fashion effects a substantial and preferably the major part of the deformation necessary to reach maximum density may sometimes be safely accompanied (in the course of achieving maximum density) by preliminary or subsequent passes or both; e. g., hot rolling passes preferably representing only a minor part of the total density change in reaching the solid-metal (or otherwise substantially uniform) density state.

For instance, in some cases it has been found possible to carry a subdensity slab of ferrous metal, having a density of about 2 to 3 gms./cc., to a density of about 6 gms./cc. in a single, major, hot rolling pass, without impairing the structural integrity of the body. In such operation, the working is found to be carried (in the single pass) beyond the range of elongations wherein the diiferential deformation is excessive, the body being thus brought to a point where the differential deformation is again safely small and where a substantial density increase has presumably occurred throughout the body. Thickness reduction to maximum density, say about 7.5

' gms./ cc. can then be completed with one or more further passes; e. g., hot rolling passes. It also appears possible in some cases to employ a preliminary hot pass or passes on the unworked subdensity slab before proceeding to reduce it throughout to substantially uniform, high density as explained in the foregoing or eleswhere herein, but such preliminary operation (which will provide'derlse skins on a low density core) should preferably keep below the limit of permissible difierential deformation (i. e. the limit explained in connection with Figure 4B) and most advantageously well below such limit.

In other words, each rolling pass or other single continuous application of deforming force on a body constituted wholly or partly of subdensity metal should preferably be controlled or designed to avoid leaving the body in a state illustrated in Figure 4C. Specifically, each single pass (depending on its purpose or its position in a sequence of passes) should preferably be such as either (a) to avoid carrying the first stage of thickness reduction beyond the point of permissible difierential deformation (Figure 4B) or (b) to carry the thickness reduction to a much further point, apparently represented by attainment of increased density throughout the body where there is likewise no excessive differential deformation. In a more general sense, an important feature of the invention is the thus exemplified concept that in rolling a subdensity metal body, the characteristics of the rolling pass or passes should be coordinated with the shape, size and density of the body to avoid excess differential deformation, such coordination being achieved by proper control of one or more of the stated characteristics or factors, indeed in efiect of all of them.

Methods of the invention are adapted to the formation of a Wide variety of products different in structural character and composition from products obtainable from conventional metal forming arts. The subdensity article formed according to the methods set forth herein may be of varied composition and/or density before being wrought. The composition is determined by the materials loaded into the shaping support for reduction to form the subdensity metal body. Such materials are loaded into the shaping support in zones or layers according to the composition desired in corresponding zones 01' layers in the final product. The density of such layers is individually controlled generally by sizing of the material therefor. The resulting subdensity body of varied metallic composition is wrought in the manner set forth herein to provide what may be termed a sandwich structure such as a low carbon steel or other ferrous metal subdensity core having integrally joined thereto, thin or thick skins or surface portions of maximum density stainless steel or other ferrous metal composition. The resulting subdensity body may also be Wrought to uniform or entire maximum density to provide, for example, a stainless' steel sheet having a mild steel core inherently bonded thereto by reason of formation therewith during the reducing process. A wide variety of compositions and densities is thus afforded in metal products of the invention. In all 'such products, the metals are contemporaneously formed in, and/or derived from, the same reducing and working processes.

In the rolling reduction of subdensity structures, a faulty or inoperative range may be experienced which can be overcome by working through the difficult range with a continuous force. The explanation of the difficult range of working by reference to a proposed theory of excessive differential deformation appears to satisfy the results of many experiments. While a differential deformation theory of explanation may be helpful to an understanding of practice of the invention, I do not wish to exclude some other explanation for the phenomena encountered in the working of subdensity bodies. Accordingly, regardless of how these phenomena may be explained, I overcome undesirable conditions by working with a continuous force through and thus travers the difficult range of rolling reduction. 1

What I claim as my invention is:

1. An article of manufacture, comprising: a body having a portion formed of subdensity metal constituted by substantially cohered particles with voids occurring therebetween and another portion of which is formed of metal substantially at maximum density hot wrought from the same subdensity body.

2. The method of forming a metal product having a maximum density portion and a portion bonded in situ thereto of lesser density, comprising hot working a part of: a subdensity ferrous metal body substantially to maximum density and to a difierential deformation less than a predetermined value.

3. The method of forming a rolled ferrous product for a maximum density portion and a portion bonded in situ thereto of lesser density, comprising hot rolling for subdensity ferrous bodies to produce a maximum density portion for an elongation characteristic of less than about ten percent.

4. The method of forming a composite metal body from metal particles to form portions of different density which comprises cohering the particles into an integrated metal structure with voids occurring between the particles and hot rolling said body to produce an outer skin portion compacted to substantially maximum density and an inner core of lesser density.

References Cited in the file of this patent UNITED STATES PATENTS 194,340 Dupuy Aug. 21, 1877 2,152,006 Welch Mar. 28, 1939 2,260,247 Darby et al. Oct. 21, 1941 2,297,817 Truxell, et a1. Oct. 6, 1942 2,598,796 Hulthen et a1. June 3, 1952 2,637,671 Pavitt May 5, 1953 7 2,686,118 Cavanagh Aug. 10, 1954 FOREIGN PATENTS 645,030 Great Britain Oct. 25, 1950 

3. THE METHOD OF FORMING A ROLLED FERROUS PRODUCT FOR A MAXIMUM DENSITY PORTION AND A PORTION BONDED IN SITU THERETO OF LESSER DENSITY, COMPRISING HOT ROLLING FOR SUBDENSITY FERROUS BODIES TO PRODUCE A MAXIMUM DENSITY PORTION FOR AN ELONGATION CHARACTERISTIC OF LESS THAN ABOUT TEN PERCENT. 